1. Field of the Invention
The present invention relates to a highly stable surface acoustic wave device which operates in a high frequency range and is produced economically.
2. Description of the Related Art
Surface acoustic wave devices utilize a surface acoustic wave which propagates with energy concentrated on a surface of a solid material. As they are compact and stable device, they are often used as an intermediate frequency filter in a TV set and the like.
The surface acoustic wave device is usually excited by applying an alternating electric field on a piezoelectric material with comb-like electrodes.
Piezoelectric material often used to form such devices include bulk single crystals such as quartz crystal, LiNbO.sub.3, LiTaO.sub.3, etc. or a ZnO thin film which is vapor deposited on a substrate.
In general, an operation frequency f of the surface acoustic wave device is determined by the equation: EQU f=v/.lambda.
in which v is a propagation velocity of the surface acoustic wave and .lambda. is a wavelength. The wavelength .lambda. is determined from a period of the comb-like electrode as shown in FIGS. 1 and 2. In FIG. 1, electrode tips each having a width d are integrally formed with a distance 3d. A pair of adjacent electrode tips comprise different electrodes and every other electrode tips comprise the same electrodes. The comb-like electrode of FIG. 1 is one of the most commonly used electrodes. The wavelength with this type of electrodes is 4d.
In the comb-like electrode of FIG. 2, two electrode tips each having a width d are repeatedly arranged with a distance 5d. The wavelength .lambda. is 8d/3. With the comb-like electrode of FIG. 2, a three-times mode is strongly excited.
The propagation velocity v depends on the piezoelectric material or the substrate material and also on the mode of surface acoustic wave.
When the single crystal piezoelectric material made of LiNbO.sub.3 is used, the propagation velocity v ranges from 3500 to 4000 m/sec., and when LiTaO.sub.3 material is used, the propagation velocity v ranges 3300 to 3400 m/sec. When the piezoelectric material comprising the ZnO thin film formed on the glass plate is used, the propagation velocity v is at most 3000 m/sec.
To increase the operation frequency f, the propagation velocity v is increased and/or the wavelength .lambda. is decreased. However, the propagation velocity v is limited by the characteristics of the material. The period size of the comb-like electrode has a lower limit due to limitations in the processing technique. By photolithography, the lower limit of the period size is set at 0.8 .mu.m. With an electron beam exposure, the processing to a submicron order is possible. However, as the line width becomes smaller, the yield becomes worse. That is, because of the limitation of the processing technique, the wavelength .lambda. cannot be reduced significantly.
Due to the above reasons, the operation frequency of the practically used surface acoustic wave device is at most 900 MHz.
By the way, as the frequency in a telecommunication, is increased such as satellite telecommunication or mobile telecommunication, it is increasingly required to provide a surface acoustic wave device which can be used in a high frequency range (GHz band), and such surface acoustic wave devices are being developed vigorously.
In general, to apply the piezoelectric thin film grown on the substrate in the surface acoustic wave device, when the sound velocity through the substrate is larger than that through the piezoelectric material, plural surface acoustic waves having different propagation velocities (zeroth order mode, first order mode, second order mode and so on from the wave having the smaller propagation velocity) are generated.
When the sound velocity through the substrate material is larger, the propagation velocity v becomes larger.
A prototype device comprising a substrate of sapphire through which the sound velocity is large (a velocity of transversal wave: 6000 m/sec., a velocity of longitudinal wave: 12,000 m/sec.) and a ZnO piezoelectric thin film formed thereon was produced (cf. Japanese Patent Kokai Publication No. 154088/1975). This prototype device achieved the propagation velocity of 5500 m/sec.
Since the sound through diamond has the largest velocity (a velocity of transversal wave: 13,000 m/sec., a velocity of longitudinal wave: 16,000 m/sec.), a surface acoustic wave device comprising a diamond substrate will realize a propagation velocity of 10,000 m/sec. or larger. Since the sound velocity through a diamond-like carbon is substantially the same as that through the diamond, a device comprising a diamond-like carbon substrate will realize the same propagation velocity as in case of the device comprised of the diamond substrate. Such a device is described in Japanese Patent Kokai Publication Nos. 20714/1989 and 62911/1989).
As an example of a laminated structure consisting of a diamond layer, a piezoelectric layer and electrodes, a structure similar to FIG. 3 is contemplated, which may be prepared by growing a diamond film 2 on a substrate 1 such as a silicon plate by a vapor phase growth method, forming comb-like electrodes 4 and then forming a piezoelectric layer 3 such as a ZnO layer. Since the electrodes are formed at an interface between the diamond film through which the surface acoustic wave propagates at a high speed and the piezoelectric layer, the structure of FIG. 3 is advantageous because of low loss of signals. Usually, the diamond film grown by the vapor phase growth method has an irregularity of 10% or more in relation to a thickness of the diamond film.
To increase the propagation velocity of the surface acoustic wave through the diamond film to an inherent value for the diamond, a thickness of the diamond film should be the same as or larger than a wavelength of the surface acoustic wave. That is, the diamond film should have a thickness of 0.5 to 5 .mu.m or larger for the surface acoustic wave device to be operated in a high frequency range of 1 GHz or higher. When the diamond film having such a thickness is formed by the vapor phase growth method, the as-grown diamond film has an irregularity of 0.2 to 5 .mu.m.
Since the comb-like electrode formed on the diamond film consists of a precise pattern of a metal film having a thickness of 0.8 to 5.0 .mu.m, a substrate on which the comb-like electrode is formed has preferably a surface roughness (R.sub.max) of 0.1 .mu.m or less in view of a production yield of the device. Therefore, the surface of the diamond film should be smoothened by, for example, abrasion before the formation of the comb-like electrode.
Since the diamond is extremely hard and such surface smoothening by abrasion is time and cost-consuming, a surface acoustic wave device comprised of diamond film has not been produced at a low production cost.
When a surface acoustic wave device has a structure of FIG. 4, since the piezoelectric layer 3 is formed on the diamond film 2, the layer 3 has substantially the same irregularity as the diamond film 2.